Bonded fibrous casing substrates and method of making same

ABSTRACT

A porous fibrous sheet material for use in the manufacture of food casings and the like is pressed prior to bonding and is bonded with about 10% by weight or less of a non-viscose bonding agent. The pressed and bonded material can be used to form casings having characteristics of thinness and elasticity similar to the thinness and elasticity of viscose-bonded casings, without the environmental problems associated therewith. The non-viscose bonding agent will result in significantly less shrinkage in the base web than results from viscose bonding.

This is a divisional of application Ser. No. 07/755,121 filed Sep. 5,1991 and issued as U.S. Pat. No. 5,300,319 on Apr. 5, 1994.

BACKGROUND OF THE INVENTION

The present invention relates generally to casings used for packagingfood products such as sausage and the like. More particularly it isconcerned with a non-viscose-bonded fibrous base web used in makingreinforced casing, and a method of making the non-viscose-bonded fibrousbase web.

Heretofore it has been the practice to make reinforced films, tubing,casings or skins for food products and the like by the encasement ofbonded fibrous base papers or substrates in a film forming material. Thesubstrate must have good absorptivity to provide for proper penetrationof the encasing material. In order to withstand the treatment conditionsat the time of encasement, the substrate must possess substantial wetand dry strength. Furthermore, when a caustic encasing medium such asviscose is used, the substrate must have sufficient caustic strength toenable it to retain its structural integrity during casing-formingoperations. Heretofore substrates of this type frequently have beenprepared by bonding a preformed and dried paper or fibrous base web witha dilute (1%) viscose solution followed by the steps of drying,regenerating the cellulose, washing and redrying. This bonding operationusing the dilute viscose solution is sufficient to impart to thesubstrate adequate caustic resistance, and to allow the substrate toretain its porous, absorbent characteristics in order to result incomplete impregnation and encasement by the concentrated viscosesolution. Typically the casing-forming operation includes the steps offorming the substrate into a cylindrical tube, impregnating and encasingthe substrate tube with a highly caustic viscose solution, regeneratingthe impregnate with acid, washing to remove excess acid and viscose, anddrying the final reinforced film or casing. This process is set forth ingreater detail in U.S. Pat. No. 3,135,613, Underwood, entitled"Impregnated Paper Webs and Method of Making Sausage Casing Thereof",thus clarifying the sequential evolution of the base web through thebonded substrate phase and then into the reinforced casing.

The casings produced in the manner set forth possess sufficientelasticity and burst resistance to be particularly well suited forenclosing meat and other food products that are injected into theinterior of the tubes under pressure. Furthermore, they are sufficientlythin to allow proper drying of the tube during manufacture, and toprovide for good through-sheet viscose penetration under rapidencasement operations, favorable shirring characteristics, andacceptable clarity of appearance. The viscose-bonded, viscose-encasedcasings thereby provide firm, uniform enclosures for well known productssuch as sausage, bologna and the like as well as other food products.

However, due to the environmental concerns associated with the use ofviscose, various patents subsequent to the aforementioned U.S. Pat. No.3,135,613 are directed toward the use of alternative materials forbonding the paper webs to provide appropriate casing substrates. Inselecting bonding materials other than the commercially employedacid-regenerated dilute viscose, it is important that the bondingmaterials meet both the processing and performance requirements of thefood casings to be produced therefrom. The substrate must retain itsporous, absorbent characteristics in order to permit completeimpregnation and encasement by the concentrated viscose solution. Thisis necessary in order to impart the required specific physicalproperties to the finished tube which make it an effective casing, orother end-use product. These physical properties include adequatestrength and tube stretch during the stuffing, cooking and curingoperation to ensure that the casing expands (or shrinks) to the requireddiameter, necessary clarity of appearance and a lack of unsightlyartifacts. In addition, the initial base substrate must be thin enoughto allow for proper drying of the tube during manufacture and forsufficient through-sheet viscose penetration under dynamic conditions.The substrate also should result in a casing having favorable shirringand other convertibility characteristics.

The characteristics described above have been embodied in thetraditional viscose binder. The alternate binders that have been tried,however, have met with varying degrees of success. While certain of thechemical characteristics of viscose have been mimicked by alternativebinders, viscose has thus far been unique in the magnitude of itsthree-dimensional shrinkage during regeneration, producing a uniquelythin sheet while maintaining its extensibility and other above-mentionedcharacteristics without further treatment. The use of anon-viscose-bonded substrate which is generally thicker than theconventional viscose-bonded substrate can result in poor viscosepenetration under rapid encasement processes as well as poor shirringcharacteristics of the final casing, and can lead to drying problemsand/or the appearance of artifacts on the casing. Furthermore, whenalternative binders are used, in order to compensate for the lack ofshrinkage and the resulting thicker paper product, it may be necessaryto reconfigure or modify the equipment that is used during encasement,or to alter the encasement process itself such as by decreasing viscoseviscosity or increasing dwell times, the latter reducing productivity.Furthermore, a casing formed from a viscose-bonded sheet has acharacteristic high extensibility (as the tube is being expanded) whichgenerally cannot be matched by alternate bonding systems. Thus, areconfiguration or modification of equipment used in stuffing operationsmay be required to accommodate a non-viscose-bonded casing, or, in orderto expand to the same final diameter as the naturally stretchyviscose-bonded casing, the paper might have to have a larger initialwidth than a viscose-bonded casing paper. Such reconfiguration ormodification of converting equipment can be expensive and inconvenient,requiring shut-down of operations.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a bonded fibrous basesubstrate which has the characteristics heretofore associated with thedilute (1%) viscose-bonded material, but does not have the environmentalproblems associated with viscose. Included in this object is theprovision of a bonded fibrous base substrate which is useful to makefood casings and the like which are thin and have good elasticity,acceptable strength, and minimal discoloration.

Another object of the invention is to provide a new and improved methodof producing a bonded fibrous base substrate which does not containviscose and is useful to make food casings.

Yet another object of the invention is to provide a pre-pressed,non-viscose-bonded substrate which can be substituted directly for aviscose-bonded substrate in the manufacture of food casings, requiringlittle or no modification of the equipment or processes used forencasement, or for stuffing operations.

Other objects will be in part obvious and in part pointed out more indetail hereinafter.

These and related objects and advantages of the invention are realizedby providing a pre-compressed, bonded porous fibrous sheet materialwhich does not contain viscose, the sheet material being useful for themanufacture of food casings and the like. The sheet material contains afibrous base web which is pre-pressed, i.e., pressed before it isbonded. The pre-pressed web is bonded with about 10% by weight or lessof a non-viscose bonding agent which is characterized by its ability toimpart less shrinkage to the bonded sheet material upon drying than theshrinkage imparted by a viscose bonding agent. The pre-pressed, bondedfibrous base web, which preferably has a thickness of no more than about135% of the thickness of a viscose-bonded sheet having a correspondingbasis weight, can be used to manufacture food casings and the like whichhave characteristics of thinness, elasticity, strength and clarity whichrender them useful as substitutes for casings formed from viscose-bondedsheet material. Preferably the bonded substrate is used to form a casinghaving an elasticity comparable to the elasticity of a casing formedfrom a viscose-bonded sheet material which has a corresponding basisweight after bonding.

The sheet material of the invention is formed by providing a base webmaterial which preferably will result in a bonded sheet having a basisweight of about 15-35 grams per square meter, pre-pressing the base webmaterial to a pressed thickness which is not less than the finalthickness of the treated sheet after drying, treating the pressed baseweb material with a non-viscose bonding solution effective to bond thebase web material, the bonding agent being characterized by its abilityto impart less shrinkage to the bonded sheet material than the shrinkageimparted by a viscose bonding agent, and then drying the pressed andtreated web. Due to the step of pressing the web prior to bonding, thenon-viscose-bonded, dried web is useful to form casings which havecharacteristics of thinness and elasticity comparable to the thinnessand elasticity of casings formed from viscose-bonded webs which havecorresponding basis weights after bonding.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others and thearticle possessing the features, properties, and relation of elementsexemplified in the following detailed disclosure.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, a porous bonded fibrous base webhaving the foregoing and related advantages can be achieved bypre-pressing an unbonded fibrous base web to a predetermined thickness,and then subsequently bonding it with a bonding agent which comprises afilm forming material that will impart to the base web substantialstrength while permitting the rapid penetration of that solution intothe substrate structure. The combination of the pre-pressing step andsubsequent bonding with the film forming material results in a base webthat can be used to form a casing having properties of thinness andelasticity comparable to the thinness and elasticity obtained whenconventional viscose binder is used without pressing. The base webmaterial of the invention can be used to make a variety of food casingsand the like, including casings that are formed without necessitatingsubstantial alterations in the equipment or techniques that are employedin conventional methods of encasing viscose-bonded substrates with ahighly caustic viscose solution. The base web material of the inventionalso can be used in conjunction with encasement processes which do notuse viscose as the encasing medium.

Briefly, the process of the invention preferably comprises the steps offorming a dilute suspension of cellulosic fibers, such as manila hempfibers, and thereafter forming a fibrous base web from the suspension.The base web is pre-pressed to a desired thickness which is not lessthan the final thickness of the sheet material, is dried, issubsequently bonded using a bonding agent which will impart lessshrinkage to the bonded sheet material upon drying than the shrinkageimparted by a viscose bonding agent and will result in a base web havingsufficient strength and absorptivity to be used in the formation of acasing, and is dried again. A one-step or multi-step bonding process canbe used.

The base web is generally composed of the natural fibers of purecellulose and preferably comprises the long, light weight andnonhydrated fibers of the Musa Textilis species, typical of which arehemp fibers. Webs made from this material are generally soft, porouspapers of uniform texture and thickness and possess tensile strengthratios close to unity, that is, a substantially equal tensile strengthin both the machine and transverse direction. However, it will beappreciated that the tensile ratio may vary from about 0.5 to about 1.5where such is desired. Webs also can include other natural fibers suchas sisal or wood pulp, and synthetic fibers such as rayon.

The base web can be pre-pressed using any technique which willeffectively reduce the thickness of the paper a predetermined amount,e.g., to a pre-bonded thickness that will result in a bonded, driedsubstrate having a "target" thickness which is comparable to thethickness of a viscose-bonded web. The pressing may be conducted in asingle stage or multi-stage process. Preferred pressing methods includewet pressing and calendering. However, other pressing techniques whichachieve the desired result also are within the scope of the invention.Wet pressing preferably is conducted prior to drying the web, butalternatively can be conducted after rewetting a dried web, in whichcase a size press, or a series of size presses, can be used. Duringconventional wet pressing, the sheet is transferred between two rollshaving predetermined hardness and camber which are loaded on the ends toprovide the required pressing pressure. In addition, the sheet isusually supported by felts on one or both sides to absorb the waterremoved from the web. Typical wet press loads for a web having a basisweight of 15-35 grams per square meter for a single stage pressing stepare 5-200 pounds per lineal inch (p.l.i.), however, higher or lowerpressures may be required. The load will depend upon the moisturecontent of the paper fibers, the type of fiber, subsequent bondingagent, and encasing material, the desired properties of the finalcasing, and the type of pressing equipment that is used. According tothe invention, the preferred pressure for wet pressing is within therange of 25-75 p.l.i., and more preferably is about 25-60 p.l.i. Whenthe web is calendered using a conventional calendar, the pressurerequired to achieve a desired reduction in sheet thickness is likely tobe substantially higher than that applied in wet pressing, because thepaper is dry. For example, calendering pressures typically range from 75to 700 p.l.i., preferably 85-400 p.l.i., and more preferably 90-300p.l.i. Size pressing ordinarily will require more pressure than wetpressing in order to achieve the same final thickness because themoisture content of the fiber passed into a size press is usually lowerthan the moisture content of fiber sent into a wet press.

With any pressing technique that is used, it is important that thepressing occur prior to bonding in order to result in a casing having anelasticity (stretch) comparable to that of a casing formed from aviscose-bonded substrate. While the relationship between the step ofpressing the web before bonding and the result of good elasticity of thecasing is not fully understood, it is believed that pressing beforebonding does not reduce the elasticity of the material because it doesnot interrupt binder-to-fiber bonding. On the other hand, pressing afterbonding can interrupt binder-to-fiber bonding, resulting in a reductionin elasticity of the casing product. It is also possible thatpre-pressing provides less interruption to fiber-to-fiber bonding thanthe interruption that occurs when pressing is conducted after bonding.

According to a preferred embodiment of the invention, the web is pressedto a pre-bonded thickness that will result in the production of abonded, dried substrate having a desired "target" thickness. Thethickness to which the pre-bonded web is pressed depends upon the degreeto which the web will shrink (or expand) as a result of application ofthe bonding agent. Example 1 below shows the shrinkage of unpressed websbonded with a variety of different binders, some of which cause greatershrinkage of the web than others. For commercial production of casings,it often is necessary to obtain a bonded substrate having a very precisethickness, e.g., a value within several microns of the target thickness.The appropriate pre-bonding thickness which will result in a substrateof the "target" thickness is preferably determined empirically, as theamount of shrinkage of the substrate upon drying after bonding willdepend in part upon the degree to which the web is pressed beforebonding.

Typical thicknesses of bonded, dried substrates produced according tothe invention range from about 55-150 microns, preferably 60-130microns, and more preferably 70-120 microns. The thicknesses of thesubstrates before bonding can be up to at least about 200 microns.According to the invention, when a pre-pressed, non-viscose-bondedsubstrate is to be substituted for a viscose-bonded substrate, and issubsequently to be encased with a suitable encasing material, such asviscose, it is preferred that the thickness of the bonded, driedsubstrate is no more than about 135% of the thickness of aviscose-bonded substrate, more preferably is no more than about 120%,and most preferably no more than about 110% of the thickness of aviscose-bonded substrate. Similar percentages are likely to apply whennon-viscose encasement processes are used. Ideally, when the non-viscosesubstrate is to be substituted directly for a viscose-bonded substrate,the web should be pressed to a thickness that will result in a dried,bonded substrate having a thickness within about 15 microns, morepreferably within about 10 microns, and most preferably within about 3microns of the target thickness for the viscose-bonded substrate.

As indicated above, the pre-pressed, non-viscose-bonded substrate musthave sufficient strength in order to serve as a substitute for aviscose-bonded substrate. It is noted that the threshold value ofstrength appears to be met without difficulty using the binders of theinvention. Preferably the bonding agent should improve the secureadhesion of the casing forming material to the reinforced substratesince it is believed that secure bonding therebetween contributes to theburst strength of the resultant casing. At the same time, it should bekept in mind that the web should be devoid, at least as far as possible,of impregnates that might interfere with both the absorption and bondingmechanism. It is therefore necessary that the bonding agent utilizedcause as little resistance as possible to the penetration of the casingforming coating into the reinforcing substrate material.

Although the bonding agent can be applied to the base web using variousstandard application techniques, it is generally preferred that thebonding agent be incorporated into the base web material by immersingthe web in an aqueous solution of the bonding material and subsequentlydrying the treated web. In other words, it is generally preferred thatthe bonding be carried out by dip coating in the aqueous binder solutionto apply an appropriate coating level of the binder material thereon.The dip coating can be applied by any conventional method, such as byusing a size press with relatively low pressure. The treated web is thendried after each bonding stage, or is otherwise treated to fix thebinder in the web.

As mentioned, the bonding can be effected in a one-step or multi-stepprocess. In a one-step process, a film forming bonding agent can beemployed alone, however, it preferably is combined with aninsolubilizing or cross-linking agent which provides improved physicaland chemical properties during the casing forming operation. A preferredbonding agent for a one-step process is a mixture of poly(vinyl alcohol)as a film-forming material, and a polyamide epichlorohydrin as across-linking agent.

Multi-step processes are preferred over one-step processes when thecompounds which are used to effect bonding cannot effectively be mixedin a single solution, or when certain desired properties cannototherwise be achieved. Multi-step processes generally involve the use offilm forming materials and/or cross-linking agents applied in successivestages. Generally, after each step of a multi-step bonding process, theweb is dried. A wide variety of multi-step processes can be used. One ofthe two-step processes which has been found particularly useful when thesubstrate is to be subsequently encased in viscose involves, as a firststep, treating the base web with a film forming bonding agent, and afterdrying the web, treating the web with a mixture of a film formingbonding agent and a cross-linking agent or an insolubilizing agent. Athree-step process which provides particularly favorable resultsinvolves treating the base web with a cross-linking agent andsubsequently drying the web, treating the web with a film forming agent,followed by a second drying stage, and, thirdly, treating the base webwith an insolubilizing agent.

The film forming materials preferably used according to the presentinvention are thermoplastic and/or thermosetting polymers, as well asother polymers and their corresponding salts. Preferred film formingmaterials include poly(vinyl alcohol), chitosans, i.e., de-acetylatedchitins, including polymers formed by the reaction of chitin (a highmolecular weight linear polymer of anhydro-N-acetyl-glucosamine) withconcentrated alkali, such as those sold under the trade name "SeaSanMerN2000" by CTC Organics of Atlanta, Ga., polyacrylamides, alginates, suchas sodium alginates sold by Merck & Co., Kelco Division under the tradename "Kelgin", cellulose based materials such as carboxymethylcellulose, methyl cellulose, hydroxy ethyl cellulose, hydroxypropylmethyl cellulose, and hydroxypropyl cellulose, cationic and anionicstarches, acrylic latexes, modified proteins such as casein or soy,vinyl acetate ethylene emulsions, and vinyl acrylic emulsions.

The insolubilizing agents or cross-linking agents employed can bethermosetting resins curable under acid or alkaline conditions, or canfall into other categories, such as metal salts and multivalent metalion complexes. Cationic wet strength resins have proved satisfactoryfrom the standpoint of caustic resistance when used in conjunction witha film forming material, and have resulted in strengths for the casingsthat are comparable to or exceed those obtained by the previouslyemployed dilute viscose bonding treatment. The preferred insolubilizingor cross-linking agents are polymeric reaction products ofepichlorodydrin and polyamides, dialdehydes, urea formaldehyde, melamineformadehyde, and glyoxylated polyacrylamides. Useful material other thanthermosetting resins include multivalent metal ion complexes such asammonium zirconium carbonate, calcium citrate and the like, andinorganic salts such as calcium chloride, sodium tetraborate andaluminum sulfate. When epichlorohydrin is used, it preferably isincluded in amounts sufficient to convert the secondary amine groupstherein to tertiary amines. Generally polyamides from polyalkylenepolyamines and saturated or unsaturated aliphatic or aromaticpolycarboxylic acids containing from about three to ten carbon atoms arepreferred. A typical example of such a material is the water solublethermosetting cationic epichlorohydrin-polyamide reaction product soldby Hercules Incorporated of Wilmington, Del. under the trade names"Kymene 557H", "Kymene 2064", "Kymene LX" and the like. Othercommercially available agents include styrene-maleic anhydridecopolymers sold by Monsanto Plastics and Resins Company under thetrademark "Scripset", and a polyamide-type resin sold by Borden ChemicalDivision of Borden, Inc. under the trademark "Cascamid C-12".

The amount of film forming material and, optionally, cross-linking agentused in a bonding solution will vary depending on the desired propertiesof the substrate, and in some cases, will depend upon the number ofsteps in the bonding process. For any given combination of binderingredients, a useful proportion of the various components is bestdetermined empirically. When poly(vinyl alcohol) is used as afilm-forming agent, its concentration can be up to 15% by weight insolution but is usually less than 5% by weight and typically fallswithin the range of from about 0.7% to 4% by weight. The insolubilizingagents are employed as dilute aqueous solutions and each are present atconcentrations of less than about 5% by weight in the paper and, in someinstances, as low as 0.2%, either alone or in combination with a filmforming solution. Typically, when used in a one-step process, a filmformer and an insolubilizing agent each are present at solutionconcentrations between 0.2% and 4% with the ratio of insolubilizingagent to film former falling within the range of from about 1 to 10 toabout 1 to 1. As will be appreciated, the concentration of the specificmaterials utilized will depend upon the ability of that material toachieve the desired wet strength characteristics and absorption at theconcentration levels employed. Typically, when poly(vinyl alcohol) isused as a film forming agent in a one-step binding process, the amountby weight of solution of cross-linking agent will exceed 0.1% andpreferably falls within the range of 0.2-1.5% by weight, with the ratioof poly(vinyl alcohol) to cross-linking agent being within the range of1:1 and 6:1, and preferably within the range of 5:4 to 5:1. For example,the preferred poly(vinyl alcohol) to cross-linking agent ratio within abonding solution according to the invention is about 3:1 to 2:1.

In the processes described above, "poly(vinyl alcohol)" as used hereinis intended to cover solutions of vinyl polymers where the poly(vinylalcohol) moiety constitutes up to 100% of the vinyl polymer present inthe solution. Since poly(vinyl alcohol) is normally prepared byhydrolysis of polyvinyl esters such as poly(vinyl acetate), the degreeof substitution will vary and the hydroxyl content may varysubstantially. Accordingly, when the substrate is to be encased inviscose, it is generally preferred that the material exhibits therequisite effects of imparting caustic strength. This may be achieved ina one-step process at poly(vinyl alcohol) levels of 25% and less butpreferably at levels where the polymer is predominantly poly(vinylalcohol), that is, where poly(vinyl alcohol) levels are at least 50% andpreferably about 80% or greater. Although various commercial productsare available, it has been found that excellent results are obtainedwhen using a high molecular weight, (i.e., having a solution viscosityat 4 wt % solids of about 40 centipoise or greater, typically 45-70centipoise), fully hydrolyzed (98-99% hydrolysis) aqueous poly(vinylalcohol) solution such as the material sold by Air Products Co. underthe trademark "Airvol 350" or the super hydrolyzed (99+% hydrolysis)solutions sold under the trademark "Airvol 165".

In a two-step process using a film forming material in the first stepand a combination of a film forming material and a cross-linking agentin the second step, the proportion of first stage to second stage filmforming material does not appear to significantly affect the end result.Excellent results have been achieved with a ratio of first stage tosecond stage film forming material ranging from about 3 to 1 to about 1to 3.

The final casing can be made in accordance with conventional casingtechniques. In contrast to a 10% or less pickup of binder by the baseweb material, the casing forming operation results in not onlyabsorption of the casing forming solution within the substrate, but alsoresults in the complete encasement of the substrate by the film formingmaterial. Thus, the relative proportion of the casing film to thesubstrate on a weight basis is at least about 1:1, and can be as high as5:1. Thus the resultant casing product is, in effect, a film of thecasing forming material reinforced by a bonded fibrous substrate fullyembedded therein.

In a preferred embodiment of the one-step bonding process, the firststage film forming material consists of an aqueous solution of 1-4 wt %of a mixture containing poly(vinyl alcohol), epichlorohydrin (Kymene557H), and surfactant.

Preferred embodiments of two-step bonding processes include a first dipin sodium alginate (Kelgin) and epichlorohydrinpolyamide reactionproduct (Kymene 557H), and, after drying, a second dip in calciumchloride, or, alternatively, a first dip in a solution containingchitosan (for example, SeaSanMer N2000) and, after drying, a second dipin a cross-linking agent such as glyoxylated polyacrylamide. Forexample, the two-step bonding process can be a first dip in 0.5-3 wt %Kelgin XL and 0.3-1 wt % Kymene 557H, followed by a second dip in 0.2-5wt % calcium chloride.

In a preferred embodiment of a three-step bonding process, the firststage is a dip in an epichlorohydrin-polyamide reaction product (Kymene557H), followed by drying. The second stage involves treatment in a filmforming solution of sodium alginate (Kelgin XL) followed by drying, andthe third stage involves treatment in an aqueous solution of calciumchloride or other calcium salts. A particularly preferred process ofthis type involves 0.5-2 wt % Kymene 557H, 1-3 wt % Kelgin XL, and 0.2-5wt % calcium chloride.

In both one-step and multi-step bonding processes, it is advantageous toadd to the binder solution very small amounts of a surfactant as anabsorption aid. In this connection materials such as the nonionicalkylaryl polyether alcohols may be used, as well as nonylphenoxy,poly(ethyleneoxy) ethanols, block copolymers of ethylene oxide andpropylene oxide, dodecyl phenoxy poly (ethyleneoxy) ethanols,polyethylene glycol ethers, polyethylene glycol esters, ethoxylatedalkyl phenols and alcohols, cellulose ethers, polyalkylene glycols,polyoxyalkylene glycols, alkylaryl polyether alcohols, polyoxethylenesorbitan moonolaurate and monoleate, alkylaryl polyethoxy ethanols,propylene glycols, and polyethylene oxide resins. The surfactantspreferably are used in the dilute binder solution in concentrationsbelow 2% by weight and, in fact, at concentrations below 0.5% andpreferably less than 0.1% so as to avoid loss of wet strength in thebonded substrate. The type of surfactant which is used generally willnot adversely affect the properties of the final casing product as longas it is used in an appropriate amount. Typically, surfactants are usedin concentrations of about 0.01 to 0.05% by weight. Below this level,the water climb characteristics of the substrate are not positivelyaffected.

In one-step and multi-step processes, the coated and dried substrateevidences a binder pickup of about 10% by weight or less, with theamount of binder typically falling within the range of 0.5 to 8% byweight. Best results are achieved when the binder pickup is about 2.0 to6.5% by weight of the bonded substrate. The bonded substrate preferablyretains a high degree of its porous, absorbent character in order topermit impregnation and encasement during the final casing formingoperation. Generally the porosity of the pre-pressed and bondedsubstrate can be measured in accordance with TAPPI test methodT251-pm-75 and exhibits a Gurley porosity greater than 275liters/minute. The porosity will vary with the weight of the pre-pressedand bonded base web, and typically falls within the range of about 300to 1000 liters/minute. Lighter sheets will of course have a higherporosity while heavier weight materials exhibit lower porosities. Thethickness of the dried, bonded substrate will depend upon the thicknessand type of base web which is used, the degree to which the web ispressed, the pressing technique, the type and quantity of binder, andthe bonding technique. Other conditions such as the humidity of theenvironment also effect web thickness.

A positive correlation between the suitability of a substrate forencasement and the wet and dry strength, caustic strength, wet and dryelongation, and water climb of the substrate is not known to exist.However, it is generally, although not always, the case that a bonded,fibrous substrate having, by way of example, a basis weight of 23.7g.s.m. and a relatively low level of binder add-on has across-directional wet strength of about 500 g/25 mm or more, across-directional dry strength of about 2000 g/25 mm or more, and awater climb of 50 seconds or less. When the bonded substrate is to beencased in viscose, the substrate often, although not necessarily,requires a caustic strength of about 300 g/25 mm or more. Higher valuesprobably would be obtained for substrates having higher basis weightsand/or containing larger amounts of binder. Lower values may be obtainedfor substrates having lower basis weights and/or smaller amounts ofbinder.

As mentioned, the pre-pressed, non-viscose-bonded substrate of theinvention can be used to form a casing product having properties ofelasticity, thinness, burst strength and clarity of appearancecomparable to those of a casing made from a viscose-bonded substrate.Therefore, in many instances, the substrate permits stuffing and furtherprocessing to be conducted without requiring modification of thepre-stuffed size of the casings. The measured burst strength of a casingwill depend upon many factors, including the composition of thesubstrate and encasing medium, the diameter of the casing, and themethod used to measure burst strength. For example, for a casing formedfrom a viscose-bonded substrate of 25.4 grams per square meter (g.s.m.)of 100% hemp fiber having a diameter of about 60 mm, the burst strength,measured in the laboratory as in Example 2 below, is likely to be atleast about 9 p.s.i. Frequently, it is necessary to achieve nearly thesame degree of burst strength and elasticity in a casing formed from anon-viscose-bonded substrate as is obtained when a casing is formed froma substrate bonded with viscose. However, the range of acceptable valuesof burst strength and elasticity will depend upon the intended use ofthe non-viscose-bonded casing. Generally, for a meat casing formed froma non-viscose-bonded fibrous substrate which is to be substituteddirectly for a viscose-bonded substrate, a burst strength of at least80% of the burst strength of a meat casing made from a viscose-bondedcasing is preferred, and a burst strength of at least 90% of the burststrength of a viscose-bonded casing is particularly preferred.Furthermore, an elasticity which is at least 90%, preferably at least95%, and more preferably at least 98 % of the elasticity of aviscose-bonded casing is desired. Casings having an elasticity withinabout 99.5% of the elasticity of a casing formed from a viscose-bondedsubstrate of the same basis weight have been found to be particularlyuseful as substitutes for casings formed from viscose-bonded substrates.However, the required minimum values of elasticity and burst strengthwill vary depending upon the amount of stretch to be applied to thecasing during the stuffing operation.

It is noted that the process of the invention is effective for resultingin a finished casing regardless of the encasing medium. Thus, whileconventional viscose can be used as the encasing medium, othercompounds, including without limitation poly(vinyl alcohol) andcellulose carbamate can be used with or without cross-linking andinsolubilizing agents.

As will be appreciated the characteristics of the bonded substrate alsorender the substrate suitable for use as teabag paper, particularly forherbal teas.

Having generally described the invention, the following examples areincluded for purposes of illustration so that the invention may be morereadily understood and are in no way intended to limit the scope of theinvention unless otherwise specifically indicated. All amounts are on aweight basis unless otherwise specified.

In the series of Examples set forth, the base fibrous web materialconsisted of 100% hemp fiber sheet material.

EXAMPLE 1

This example illustrates the difference in shrinkage imparted to anunpressed substrate bonded with viscose, as contrasted to the shrinkageof unpressed substrates bonded with various non-viscose bonding agents.

In parts A-E set forth below, the base fibrous web material consisted ofmachine formed sheets having a basis weight of 25.4 grams per squaremeter. Each sheet was approximately 8 inches by 10 inches. The bondingagent was applied to the sheets in the laboratory.

Part A

A dry, unbonded sheet of the standard base web material having athickness of 145 microns was dipped in an aqueous bonding solutioncontaining 1% viscose. The sheet was subsequently dried, regenerated,washed and redried in a conventional manner. The thickness of the sheetafter redrying was 103 microns. The percent change in sheet thickness isrecorded in Table I.

Part B

A dry, unbonded sheet of the standard base web material having athickness of 143 microns was dipped into an aqueous bonding solutioncontaining 2% poly(vinyl alcohol) (Airvol 165), 0.5%epichlorohydrin-polyamide reaction product (Kymene 557H), and 0.04%surfactant, and was subsequently dried. The thickness of the sheet afterdrying was 132 microns. The percent change in sheet thickness isrecorded in Table I.

Part C

A dry, unbonded sheet of the standard base web material having athickness of 146 microns was treated as in Part B, except that adifferent bonding solution was used. The bonding solution contained 1.7%carboxymethyl cellulose, 0.7% epichlorohydrin-polyamide reaction product(Kymene 557H) and 0.04% surfactant. The thickness of the sheet afterdrying was 128 microns. The percent change in sheet thickness isrecorded in Table I.

Part D

A dry, unbonded sheet of the standard base web material having athickness of 146 microns was dipped into a first aqueous bondingsolution of 2% sodium alginate (Kelgin XL), combined with 20% sodiumhexamethaphosphate based on the weight of sodium alginate as asequestrant, and 0.7% epichlorohydrin-polyamide reaction product (Kymene557H). The sheet was dried, subsequently was dipped into an aqueousinsolubilizing solution containing 1% calcium chloride, and was driedagain. The thickness of the sheet after the final drying step was 127microns. The percent change in sheet thickness is recorded in Table I.

Part E

A dry, unbonded sheet of the standard base web material having athickness of 145 microns was treated as in Part D, with the exceptionthat the first bonding solution consisted of an aqueous solution of 0.5%chitosan (SeaSanMer N2000) and the second cross-linking solutionconsisted of an aqueous solution of 1.2% glyoxylated polyacrylamide(Parez 631NC). The thickness of the sheet after drying was 119 microns.The percent change in sheet thickness is recorded in Table I.

Parts A-E of Example 1 show that viscose-bonded paper exhibits asubstantially higher degree of shrinkage in thickness than non-viscosebinders. Poly(vinyl alcohol) exhibits the least amount of shrinkage ofthe binders shown. Thus, it is likely that a web to be bonded withpoly(vinyl alcohol) will require more pressing than a web bonded withany of the non-viscose binders used in Parts C-E to the same pre-driedthickness in order to achieve a dried substrate having a particulartarget thickness.

                  TABLE I                                                         ______________________________________                                        Comparative Shrinkage of Bonded Paper                                                                % Decrease in                                          Bonding Solution       Thickness                                              ______________________________________                                        A      Viscose             28                                                 B      2% poly(vinyl alcohol)                                                                             8                                                        0.5% cross-linking agent                                                      0.04% surfactant                                                       C      1.7% carboxymethyl cellulose                                                                      12                                                        0.7% cross-linking agent                                                      0.04% surfactant                                                       D      2% sodium alginate  13                                                        0.7% cross-linking agent                                                      followed by                                                                   1% CaCl.sub.2                                                          E      0.5% chitosan       18                                                        followed by                                                                   1.2% glyoxylated polyacrylamide                                        ______________________________________                                    

EXAMPLES 2-10

Examples 2-10 illustrate the favorable property of elasticity of casingsformed from substrates that are pressed prior to bonding with a bondingsolution. Furthermore, Examples 2-10 show that pressing a substrateprior to bonding does not adversely affect the burst strength of a tubeformed from the bonded substrate. It is noted that each substrate thatwas pressed prior to bonding had acceptable clarity of appearance.

EXAMPLE 2

As a control, a machine-formed, unbonded sheet of the standard basefibrous web material having a basis weight of about 25.4 grams persquare meter after treatment with binder was bonded using an aqueoussolution containing 1% by weight viscose. After regenerating, washingand drying, the sheet was encased on one side with viscose in thelaboratory. The substrate was encased using a single-side encasementprocess which involved cutting the bonded paper to a length of 121/2inches in the machine direction and a width of 8 inches in the crossdirection. An edge of the sheet of paper in the cross direction was thentaped to the top edge of a smooth glass plate. A bead containing 7%viscose was poured onto the paper at the tape line, and was drawn downin the machine direction using a stainless steel rod, thereby forming afilm of viscose about 0.4-1.5 mm thick on the paper. A portion of thecoated sample was cut from the tape, pulled off the glass, and wrappedaround a mandrel having diameter of 57-59 mm. The tube and mandrel wereplaced in a coagulation bath for ten minutes, removed, and then placedin a regeneration bath for ten minutes. The tube was then quicklyrinsed, removed from the mandrel, washed and dried.

The casing product was tested for percent change in tube diameter from arelaxed state to the point of burst (elasticity), and for burststrength. The test properties are set forth in Table II.

EXAMPLE 3

A machine-formed, unbonded sheet of standard base fibrous web materialwas wet pressed, dried, and was subsequently bonded with a bonding agentconsisting of an aqueous solution containing 2% by weight poly(vinylalcohol) (Airvol 165), 0.5% by weight epichlorohydrin-polyamide reactionproduct (Kymene 557H), and 0.04% surfactant. The bonded sheet had thesame basis weight as the bonded sheet used in Example 2. The sheet wasdried, and subsequently was encased in viscose on one side in the samemanner as the viscose-bonded substrate of Example 2. The casing productwas dried and was tested for percent change in tube diameter to burstand burst strength. The test properties are set forth in Table II. Theelasticity of the casing formed from a pre-pressed, poly(vinyl alcohol)bonded substrate was even higher than the elasticity of the controlcasing formed from a viscose-bonded substrate in Example 2. The burststrength of the casing also was higher than the burst strength of thecontrol casing of Example 2.

EXAMPLE 4

The process of Example 3 was repeated with the exception that instead ofbeing wet pressed the sheet was dried and calendered before it wasbonded. The test properties of the casing are shown on Table II. Theelasticity and burst strength of the encased tube formed from apre-pressed, poly(vinyl alcohol) bonded substrate were higher than theelasticity and burst strength of the control tube of Example 2.

EXAMPLE 5

The process of Example 4 was repeated with the exception that instead ofbeing calendered, the sheet was dried and was then pressed wet afterdrying and before bonding. As shown in Table II, the percent change indiameter of the tube to the point of bursting was almost as large as thecorresponding change of the control tube of Example 2. Furthermore, theburst strength was high enough to render the casing useful as asubstitute for the control tube of Example 2.

EXAMPLE 6

The process of Example 4 was repeated with the exception that thecalendering step took place after bonding instead of before bonding. Thetest properties are set forth in Table II. It is noted that theelasticity of the tube formed from a substrate that was calendered afterbonding is substantially lower than the elasticity of the control tubeof Example 2, rendering the tube unacceptable as a substitute for thecontrol tube.

EXAMPLE 7

The process of Example 3 was repeated with the exception that the wetpressing step occurred after bonding instead of before bonding. The testproperties are set forth in Table II. The tube had lower elasticity thanthe control tube of Example 2, rendering it unacceptable as a substitutefor the control tube.

                                      TABLE II                                    __________________________________________________________________________    Comparative % Change in Diameter and Burst Strength                           of 25.4 gsm Paper Coated On One Side                                                           Initial                                                                            Diameter                                                                           Change in    Burst                                 Example                                                                            Pressing                                                                            Bonding                                                                             Diameter                                                                           at Burst                                                                           Diameter                                                                            % Change                                                                             Strength                              Number                                                                             Technique                                                                           Agent (mm) (mm) (mm)  in Diameter                                                                          (psi)                                 __________________________________________________________________________    2    None  Viscose                                                                             61.1 69.4 8.3   13.6   15.9                                  3    Wet Press                                                                           poly(vinyl                                                                          64.7 75.1 10.4  16.1   16.4                                       before                                                                              alcohol mix                                                             drying and                                                                    bonding                                                                  4    calendar                                                                            poly(vinyl                                                                          57.0 66.3 9.3   16.3   16.0                                       after alcohol mix                                                             drying but                                                                    before                                                                        bonding                                                                  5    Wet press                                                                           poly(vinyl                                                                          63.8 71.7 7.9   12.4   14.8                                       after alcohol mix                                                             drying and                                                                    before                                                                        bonding                                                                  6    calendar                                                                            poly(vinyl                                                                          64.2 70.5 6.3    9.8   14.7                                       after alcohol mix                                                             bonding                                                                  7    Wet press                                                                           poly(vinyl                                                                          59.5 66.3 6.8   11.4   13.7                                       after alcohol mix                                                             bonding                                                                  __________________________________________________________________________

EXAMPLE 8

As a control, a casing was made from an unpressed, viscose-bondedsubstrate having a thickness comparable to the sheet of Example 1, PartA. The encasement process was identical to the single-side encasementprocess of Examples 2-7 with the exception that the sheet was furtherencased with viscose on a second side in the laboratory. In order tocoat the second side of the paper, i.e., the side next to the plate, athin film of viscose was drawn down the glass plate, the bonded paperwas then placed on the film, and the second thin film of viscose wasdrawn down the outer side of the sheet. The casing was tested forpercent change in tube diameter until burst and burst strength. The testproperties are set forth in Table III.

EXAMPLE 9

A sheet of standard base fibrous web material having the same basisweight after bonding as the web material of Example 8 was wet pressedand was subsequently bonded by dipping it in a solution of 2% poly(vinylalcohol) (Airvol 165), 0.5% epichlorohydrin-polyamide reaction product(Kymene 557H), and 0.03% surfactant. The thickness of the pressed andbonded sheet after drying and before encasement was substantially thesame as that of Example 8. The test properties of the encased sheet areset forth in Table III. The casing had even higher values of elasticityand burst strength than the control casing of Example 8.

For purposes of comparison, a casing was formed from an unpressedsubstrate bonded with the polyvinyl alcohol mixture. While the casingwas found to exhibit values of elasticity and burst strength comparableto those of a viscose-bonded casing, the thickness of the unpressedsubstrate was too high to render the casing useful as a substitute forthe control casing, as it would result in poor viscose penetration uponencasement and the presence of unsightly artifacts.

EXAMPLE 10

The process of Example 9 was repeated with the exception that instead ofwet pressing the sheet before bonding, the sheet was bonded, dried, andthen calendered. The calendered sheet had a thickness comparable to thatof the control of Example 8. The test properties are set forth in TableIII. This example shows that when pressing occurs after bonding, theelasticity of the final casing is insufficient to render the casinguseful as a substitute for a casing formed from a viscose-bondedsubstrate.

                                      TABLE III                                   __________________________________________________________________________    Comparative % Change in Diameter and Burst Strength                           of 25.4 gsm Paper Coated on Two Sides                                                          Initial                                                                            Diameter                                                                           Change in    Burst                                 Example                                                                            Pressing                                                                            Bonding                                                                             Diameter                                                                           at Burst                                                                           Diameter                                                                            % of Change                                                                          Strength                              Number                                                                             Technique                                                                           Agent (mm) (mm) (mm)  in Diameter                                                                          (psi)                                 __________________________________________________________________________    8    None  1% Viscose                                                                          59.2 65.1 5.9   10.0   15.1                                  9    Wet press                                                                           poly(vinyl)                                                                         59.2 65.7 6.5   11.0   17.8                                       before                                                                              alcohol mix                                                             drying and                                                                    bonding                                                                  10   calender                                                                            poly(vinyl)                                                                         59.3 64.2 4.9    8.3    14.75                                     after alcohol mix                                                             bonding                                                                  __________________________________________________________________________

As will be apparent to persons skilled in the art, various modificationsand adaptations of the structure above described will become readilyapparent without departure from the spirit and scope of the invention,the scope of which is defined in the appended claims.

What is claimed is:
 1. A pre-compressed and bonded sheet material thatdoes not contain viscose, the sheet material being suited for use as asubstrate in the manufacture of food casings and comprising a porousfibrous sheet material bonded with about 10% by weight or less of anon-viscose bonding agent wherein the bonding agent is characterized byits ability to impart less shrinkage to the bonded sheet material upondrying than the shrinkage imparted by a viscose bonding agent, the sheetmaterial having a basis weight of 15-35 grams per square meter andhaving been pressed at a rate of at least 5 pounds per lineal inch priorto bonding to provide a bonded sheet material having a thickness whichis no more than 135% of the thickness of a viscose-bonded porous fibroussheet material of the same fiber composition that has a correspondingbasis weight after bonding, the sheet material being adapted to form acasing having an elasticity of at least about 90% of the elasticity of acasing formed from a viscose-bonded sheet material of the same fibercomposition that has a corresponding basis weight after bonding.
 2. Thesheet material of claim 1, wherein the elasticity of a casing madetherefrom is at least 95% of the elasticity of a casing formed from aviscose-bonded sheet material.
 3. The sheet material of claim 1, whereinthe thickness is no more than 120% of the thickness of theviscose-bonded porous fibrous sheet material.
 4. The sheet material ofclaim 1, wherein the thickness is about 55-150 microns.
 5. The sheetmaterial of claim 1, wherein the non-viscose bonding agent comprises afilm forming material selected from the group consisting of polyvinylalcohol, sodium alginate, and the reaction product of a high molecularweight linear polymer of anhydro-N-acetyl-glucosamine and a concentratedalkali.
 6. The sheet material of claim 1, wherein the bonding agentfurther comprises a cross-linking agent selected from the groupconsisting of glyoxylated polyacrylamide, the reaction product ofepichlorohydrin and a polyamide, and a bivalent metal salt.
 7. The sheetmaterial of claim 5, wherein the bonding agent further comprises asurfactant.
 8. The sheet material of claim 1, wherein the bonding agentcomprises a film forming material and a cross-linking agent.
 9. Thesheet material of claim 1, wherein the bonding agent comprises a filmforming material.
 10. The sheet material of claim 3, wherein the bondingagent comprises a film forming material and a cross-linking agent. 11.The sheet material of claim 1, wherein the sheet material has a porosityof at least 300 liters/minute and a caustic strength of at least 300grams/25 mm.
 12. A food casing comprising a fibrous substrate embeddedwithin and reinforcing a casing film, the substrate comprising apre-compressed porous fibrous sheet material that does not containviscose and which is bonded with about 10% by weight or less of anon-viscose bonding agent characterized by its ability to impart lessshrinkage to the bonded sheet material upon drying than the shrinkageimparted by a viscose bonding agent, the sheet material having a basisweight of 15-35 grams per square meter and having been pressed at a rateof at least 5 pounds per lineal inch prior to bonding to provide abonded sheet material having a dried thickness of no more than 135% ofthe thickness of a viscose-bonded, dried porous fibrous sheet materialof substantially the same fiber composition that has a correspondingbasis weight after bonding, the casing having an elasticity of at leastabout 90% of the elasticity of a casing formed from a viscose-bondedsheet material of the same fiber composition that has a correspondingbasis weight after bonding.
 13. The casing of claim 12, wherein thecasing has an elasticity of at least 95% of the elasticity of a casingformed from a viscose-bonded sheet material that has a correspondingbasis weight after bonding.
 14. The casing of claim 12, wherein thesheet material has a thickness that is no more than 120% of thethickness of a viscose-bonded porous fibrous sheet material that has acorresponding basis weight after bonding.
 15. The sheet material ofclaim 2, wherein the thickness is no more than 120% of the thickness ofthe viscose-bonded porous fibrous sheet material.
 16. The sheet materialof claim 15, wherein the thickness is about 55-150 microns.
 17. Thesheet material of claim 15, wherein the non-viscose bonding agentcomprises a film forming material selected from the group consisting ofpolyvinyl alcohol, sodium alginate, and the reaction product of a highmolecular weight linear polymer of anhydro-N-acetyl-glucosamine and aconcentrated alkali.
 18. The sheet material of claim 2, wherein thethickness is no more than 110% of the thickness of the viscose-bondedporous fibrous sheet material.
 19. The sheet material of claim 5,wherein the thickness is no more than 110% of the thickness of theviscose-bonded porous fibrous sheet material.
 20. A pre-compressed andbonded sheet material that does not contain viscose, the sheet materialbeing suited for use as a substrate in the manufacture of food casingsand comprising a porous fibrous sheet material bonded with about 10% byweight or less of a non-viscose bonding agent wherein the bonding agentis characterized by its ability to impart less shrinkage to the bondedsheet material upon drying than the shrinkage imparted by a viscosebonding agent, the sheet material having a basis weight of 15-35 gramsper square meter and a thickness which is no more than 135% of thethickness of a viscose-bonded porous fibrous sheet material of the samecomposition that has a corresponding basis weight after bonding, thesheet material being adapted to form a casing having an elasticity of atleast about 90% of the elasticity of a casing formed from aviscose-bonded sheet material of the same fiber composition that has acorresponding basis weight after bonding.